1. Field of the Invention
The present invention relates to reflective mechanisms for deflecting light through a tape in a data cartridge.
2. Description of the Related Art
Tape in a data cartridge often is provided with holes to serve as codes to the drive in which the data cartridge is to be used. For example, the holes may indicate that the drive has reached the end or beginning of the tape, is nearly at the end or beginning of the tape, or may identify the exact type and length of tape in the cartridge inserted in the drive. The tape itself normally is opaque, and drives usually are provided with a light source and light detector to detect the presence or absence of such holes. Normally, the light is positioned below the cartridge and the detector to the front of the cartridge. The light shines through the top of the cover (which typically is transparent) or through a window cut through the metal baseplate of the data cartridge. It then reflects off of a mirror to redirect it 90.degree. onto the tape. When the tape has a hole in it, the light passes through the hole and out through the front of the transparent cover, where it is detected by a photocell in the drive. In addition, the tape sometimes has multiple holes across its width, and a drive must be able to accurately determine the number of holes based on the amount or position of the light coming through the holes.
In some applications, a prism is substituted for the mirror. For example, co-pending U.S. patent application Ser. No. 08/346,777, filed Nov. 30, 1994, and titled "Prism for a Data Cartridge" describes a potential prism structure. Published European Patent Application 0 499 400 (Minnesota Mining and Manufacturing Company) describes another structure using a prism instead of a mirror, and notes various advantages to using a prism over a mirror.
In some applications, using a prism can pose a problem. As noted, usually light is directed up through the bottom of the cartridge, then angled towards the tape (by the prism or a mirror) and detected at the front of the cartridge after passing through a hole in the tape. However, at least one drive manufacturer routinely directs the light backwards, that is, first shining it through the holes in the tape, then having it angled towards the bottom of the cartridge, and then detecting it at the bottom of the cartridge. This technique works adequately when the cartridge uses a mirror to angle the light, but may cause problems if a prism is used, especially in accurately detecting whether a single hole or multiple holes are present on the tape. These problems may be further exacerbated if the light source and/or the detector are off-center from the prism.